1. Field of the Invention
This invention is in the field of conductive compositions particularly for providing electrical continuity between two surfaces of a printed circuit board. The new composition can also be used as a connecting material between conductive circuit patterns, or for making a conductive pattern.
2. Description of the Prior Art
There are several prior art techniques for providing electrical connection between the individual layers of a multi-layer printed circuit board. In one such technique, an insulating substrate composed of an insulator such as a phenolic resin, an epoxy resin, or the like, is covered with copper foils which are adhered on the upper and lower surfaces thereof to produce a so-called copper clad laminated board. Then, a through-hole is bored at a predetermined location of the board to pass through the board and the copper foils thereon in the vertical direction. The board is then subjected to electroless copper plating and to copper electroplating in sequence to form a copper plating layer on the inner surface of at least the through-hole to thereby electrically connect the upper and lower copper foils. An etching resist is inserted into the through-hole and at the same time the portions which are to form the desired wiring pattern are also covered with etching resists. Next, portions of the copper foils are selectively etched away to leave a multi-layer printed circuit board in which the upper and lower circuit patterns are electrically connected through the through-hole.
A second prior art technique makes use of a copper clad laminated board similar to that described, the board being first subjected to selective etching to form wiring patterns on both surfaces of the insulating substrate. Then, a through-hole is bored through the substrate at a predetermined location, the hole also passing through the wiring patterns. Then, silver paste consisting of silver powders and a binder is inserted into the through-hole to electrically connect both the upper and lower wiring patterns and produce a double-surfaced printed circuit board.
Both of the techniques described above, however, have drawbacks. For example, in the case of the first-mentioned technique, the plating processes involved are rather complicated and not particularly efficient. In addition, there is a problem of disposing of waste water from the plating bath without creating pollution. Since the copper plating is carried out on the entire surfaces of the board and thereafter the wiring patterns are selectively etched, the amount of unnecessary copper plating is rather large. Upon selective etching, the copper plating layer on the substrate can be sufficiently etched to be under-cut, so that the wiring pattern becomes thin and the pattern accuracy is diminished.
In the second-mentioned technique, in which the upper and lower wiring patterns are electrically connected by silver paint inserted through the through-hole, when a DC voltage is applied to the board under conditions of some humidity, silver is moved from a high voltage portion to a lower voltage portion and an undesirable short-circuit is caused by the electromigration tendency of the silver. The through-hole resistance is higher than that of the copper plating by a factor of about 100 times and so the circuit board is poor in high frequency characteristics. Further, since the silver paint charged into the through-hole contains a binder and solvent, the paint shrinks by heat curing, and so the circuit board lacks reliability. In addition, the use of organic solvents and the like causes poor working conditions.
In a prior application, Ser. No. 246,902, filed Mar. 23, 1981, there is described a conductive material made of a mixture consisting of gallium in liquid phase and a metal which forms a eutectic mixture with gallium, together with a metal powder (either a single metal or an alloy powder) which alloys with gallium to raise the melting point of the composition. This conductive material is in the form of a paste at its initial working temperature and thereafter alloys and sets upon the lapse of a suitable time interval. Suitable amounts of liquid gallium and solid metal which form a eutectic with gallium are mixed to provide a molten mixture. After the mixture is cooled to a working temperature, metal powders to be alloyed with the gallium are added to the mixture to prepare a paste in which the metal powder will be finally alloyed with the gallium. The eutectic forming metal can be indium, tin, zinc, bismuth, or the like. The metal powder which alloys with gallium can be pure metal such as copper, nickel, cobalt, or gold, or alloys thereof such as a copper-tin alloy and the like.
The ratio of the ingredients in the composition, consisting of gallium, the eutectic former and the metal alloying powder depend upon the kind of metal which forms the eutectic and the temperature at which the material is to be worked. Since the conductive material containing the gallium alloy consists of a pasty composition at the working temperature, it is readily inserted into the through-hole. There is no volume shrinkage after curing and the resistance value of the inserted material is very low. Thus, the conductive material described in the previous application can be used in a multi-layer circuit board and is also free from the electromigration phenomenon characteristic of silver paste. Therefore, the above-described conductive paste permits a wide variation in printed circuit designs.
It is, however, difficult to keep the conductive material containing gallium in a paste state for a long period of time. This is illustrated somewhat in the diagram of FIG. 1A where a melt 1 of gallium and tin (the eutectic metal) is mixed with a copper-tin powder 2 to be alloyed with the gallium to provide a Ga-Sn-Cu alloy conductive material in the paste state. After the lapse of time, the conductive material shown in FIG. 1A at room temperature is changed so that the gallium reacts with copper to produce gallium-copper intermetallic compounds (such as Ga.sub.2 Cu, GaCu.sub.2, and so on) which are precipitated. In consequence, the paste property is gradually lost. It then becomes difficult to coat or charge the conductive material into the required location, so its characteristics are deteriorated. When the paste property is once lost, it is difficult to restore it, and much valuable material is lost.